CSS:

Their style codes can be found in the js-image-slider.css source under the comment line:

/* ----------- navigation buttons in the thumbnails bar ---------- */

Built-in JavaScript Functions

You may have noticed some JavaScript functions in above
codes such as: imageSlider.previous(), imageSlider.getAuto(), ...
They are all the slider's built-in functions that come handy for developers to add additional functionality:

previous() - Display previous slide

next() - Display next slide

getAuto() - Get current slideOptions.autoAdvance setting

switchAuto() - Toggle the slideOptions.autoAdvance setting

displaySlide(indexNumber) - Display a slide

reload() - re-initialize the slider. It helps to build the slider on the fly (after page load), or to update the slider dynamically.